A cuttlefish, a coleoid cephalopod, moves primarily by undulating its body fins.

Mollusca is one of the most diverse groups of animals on the planet, with at least 50,000 living species (and more likely around 200,000). It includes such familiar organisms as snails, octopuses, squid, clams, scallops, oysters, and chitons. Mollusca also includes some lesser known groups like the monoplacophorans, a group once thought to be extinct for millions of years until one was found in 1952 in the deep ocean off the coast of Costa Rica.

Molluscs are a clade of organisms that all have soft bodies which typically have a "head" and a "foot" region. Often their bodies are covered by a hard exoskeleton, as in the shells of snails and clams or the plates of chitons.

A part of almost every ecosystem in the world, molluscs are extremely important members of many ecological communities. They range in distribution from terrestrial mountain tops to the hot vents and cold seeps of the deep sea, and range in size from 20-meter-long giant squid to microscopic aplacophorans, a millimeter or less in length, that live between sand grains.

These creatures have been important to humans throughout history as a source of food, jewelry, tools, and even pets. For example, on the Pacific coast of California, Native Americans consumed large quantities of abalone and especially owl limpets. However, the impact of Native Americans on these molluscan communities pales by comparison to the overharvesting of some molluscan taxa by the United States in the 1960s and 1970s. Species whose members once numbered in the millions, now teeter on the verge of extinction. For example, fewer than 100 white abalone remain after several million individuals were captured and sold as meat in the 1970s. Besides having yummy soft parts, molluscs often have desirable hard parts. The shells of some molluscs are considered quite beautiful and valuable. Molluscs can also be nuisances, such as the common garden snail; and molluscs make up a major component of fouling communities both on docks and on the hulls of ships.

On the left is a marine snail, the California Trivia (Trivia californiana). Here the mantle covers much of the shell. Note how a portion of the mantle is rolled into a tube shape to form the siphon just above the head. At the right is a restoration of one of the largest of all molluscs, the Giant Squid (Architeuthis).

They also have a very long and rich fossil record going back more than 550 million years, making them one of the most common types of organism used by paleontologists to study the history of life.

Systematics
Molluscan systematics are still in flux. As you can see from the cladogram below, there is still no agreement on some of the major relationships. The polytomies shown indicate that the question of which molluscs are the most closely related is still a matter of debate.

However, new types of data and much larger and more sophisticated analyses continue to be performed. The resolved relationships shown (such as cephalopods, scaphopods, and gastropods) are recent discoveries.

Morphology
Despite their amazing diversity, all molluscs share some unique characteristics that define their body plan. The body has a head, a foot and a visceral mass. This is all covered with a mantle (also known as a pallium) that typically secretes the shell. In some groups, like slugs and octopuses, the mantle is secondarily lost, while in others, it is used for other activities, such as respiration.

The freshwater Sinistral Pond Snail (Physella sp.) scrapes algae from the glass with its radula, the two "toothy" arcs you can see lining the mouth. Click on the photo for a closer look.

The buccal cavity, at the anterior of the mollusc, contains a radula (lost in bivalves)  a ribbon of teeth supported by an odontophore, a muscular structure. The radula is generally used for feeding. The ventral foot is used in locomotion. This foot propels the mollusc by utilizing muscular waves and/or cilia in combination with mucus.

Typically, at least in the more primitive members of each group, there are one or more pairs of gills (called ctenidia) which lie in a posterior cavity (the pallial cavity) or in a posterolateral groove surrounding the foot. The pallial cavity typically contains a pair of sensory osphradia (for smelling) and is the space into which the kidneys, gonads, and anus open.

Molluscs are coelomate, although the coelom is reduced and represented by the kidneys, gonads, and pericardium, the main body cavity which surrounds the heart.

Life history and ecology
Molluscs occur in almost every habitat found on Earth, where they are often the most conspicuous
organisms. While most are found in the marine environment, extending from the intertidal to the deepest oceans, several major gastropod clades live predominantly in freshwater or terrestrial habitats. Remarkably, one study found around 3000 species within a single locality at a coral reef in New Caledonia. In terrestrial communities, gastropods can achieve reasonably high diversity and abundance: as many as 60-70 species may coexist in a single habitat and abundance in leaf litter can exceed more than 500 individuals in four liters of litter.

Many marine molluscs emerge from their eggs as planktonic trochophore larvae, however, Sinistral Pond Snails (Physella sp.) emerge from their eggs as young snails. The whitish, jellybean-shaped organisms are ostracodes (crustaceans).

Marine molluscs occur on a large variety of substrates including rocky shores, coral reefs, mud flats, and sandy beaches. Gastropods and chitons are characteristic of these hard substrates, and bivalves are commonly associated with softer substrates where they burrow into the sediment. However, there are many exceptions: the largest living bivalve, Tridacna gigas, lives on coral reefs, and many bivalves (e.g., mussels and oysters) attach themselves to hard substrates. Some microscopic gastropods even live interstitially between sand grains.

Large concentrations of gastropods and bivalves are found at hydrothermal vents in the deep sea. Living in these or other dysoxic habitats appears to be a plesiomorphic condition for the Mollusca and several outgroups. For example, the fauna of Palaeozoic hydrothermal vent communities includes the molluscan groups Bivalvia, Monoplacophora and Gastropoda as well as the outgroups Brachiopoda and Annelida.

The adoption of different feeding habits appears to have had a profound influence on molluscan evolution. The change from grazing to other forms of food acquisition is one of the major features in the radiation of the group. Based on our current understanding of relationships, the earliest molluscs grazed on encrusting animals and detritus. Such feeding may have been selective or indiscriminate and will have encompassed algal, diatom, or cyanobacterial films and mats, or encrusting colonial animals. Truly herbivorous grazers are relatively rare and are limited to some polyplacophorans and a few gastropod groups. Most chaetodermomorph aplacophorans, monoplacophorans and scaphopods feed on protists and/or bacteria while neomeniomorph aplacophorans graze on cnidarians. Cephalopods are mainly active predators as are some gastropods, while a few chitons and septibranch bivalves capture microcrustaceans. Most bivalves are either suspension or deposit feeders that indiscriminately take in particles, but then elaborately sort them based on size and weight, typically assimilating bacteria, protists, and diatoms.

The fossil record
The Mollusca include some of the oldest metazoans known. Late Precambrian rocks of southern Australia and the White Sea region in northern Russia contain bilaterally symmetrical, benthic animals with a univalved shell (Kimberella) that resembles those of molluscs. The earliest unequivocal molluscs are helcionelloid molluscs that date from Late Ediacaran (Vendian) rocks. In the Early Cambrian the Coeloscleritophora are also present. Most of the familiar groups, including gastropods, bivalves, monoplacophorans, and rostroconchs, all date from the Early Cambrian, whereas cephalopods are first found in the Middle Cambrian, polyplacophorans in the Late Cambrian, and the Scaphopoda in the Middle Ordovician. Most of these early taxa tend to be small (10 mm in length). The Late Vendian-Early Cambrian taxa bear little resemblance to the Cambrian-Ordovician taxa (most of which remain extant today).

On the left is Inoceramus sp., a bivalve from the Cretaceous of Alameda County, CA. At right is Turritella andersoni, a gastropod from the Eocene of Ventura County, CA.

After their initial appearance, molluscan taxonomic diversity tended to remain low until the Ordovician, when gastropods, bivalves, and cephalopods show large increases in diversity. For bivalves and gastropods this diversification increases throughout the Phanerozoic, with relatively small losses at the end-Permian and end-Cretaceous extinction events. Cephalopod diversity is much more variable through the Phanerozoic, whereas the remaining groups (monoplacophorans, rostroconchs, polyplacophorans, and scaphopods) maintain low diversity over the entire Phanerozoic or became extinct.